System for burning fine-grained material, particularly for the manufacture of cement clinkers

ABSTRACT

The invention relates to a method for burning fine-grain material, particularly for the manufacture of cement clinker from cement raw meal. The material is thermally treated in a multi-stage burning process with a pre-heating stage, a calcining stage with a high-degree of calcination, a sintering stage in a very short rotary kiln and a cooling stage. Fuel is introduced both into the sintering stage in the short rotary kiln as well as into the calcinating stage. Hot exhaust air from the cooling stage is supplied both to the sintering stage as well as to the calcining stage as furnace air. The invention also relates to an apparatus for the manufacture of mineral products of the burning process such as cement clinker.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of our copending application,U.S. Ser. No. 063,102, filed Aug. 2, 1979, now U.S. Pat. No. 4,298,393,and assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to methods and apparatus for the production ofcement clinker.

2. The Prior Art

With the introduction of raw meal pre-heaters into the cement clinkerburning process, rotary kilns which up to then had been long could bebuilt shorter. With modern burning systems incorporating conventionalcyclone pre-heaters, a ratio between the length of the rotary kiln tothe inside rotary kiln diameter of approximately 15:1 through 17:1 hasbecome general in practice. Efforts of the system manufacturers tofurther reduce the relative rotary kiln length have failed because ofthe safety requirements of the system users. Longer rotary kilns havetended to result in disruption-free operation, particularly with respectto interior cycles.

With the introduction of pre-calcination technology with secondaryfirings in the pre-heating system, a high degree of deacidification ofthe raw meal was achieved before entry into the rotary kiln. Theadvantages of lower heat introduction in the sintering zone of therotary kiln, however, were only partially exploited. The kiln diameterwas approximately reduced in relationship to the proportionate heatamounts supplied, on the one hand, to the pre-burner locations and, onthe other hand, to the sintering zone of the rotary kiln. Until now, thestandard ratio of kiln length to inside kiln diameter (15:1 through17:1) mentioned above was approximately retained. As a result, specificrotary kiln lengths which were still large ensued, which resulted inhigh investment and operating costs. On the other hand, a reduction ofthe diameter of the rotary kiln, given the same throughput capacity andthe same degree of oven charge, produced a shorter dwell period of theraw meal or, respectively, clinker in the rotary kiln. Therefore, untilnow, a further shortening of the rotary kiln was not considered.

The substances volatilizing in the rotary kiln often exhibit harmfulcomponents such as alkali compounds and sulfur which, upon theircondensation from the rotary kiln exhaust gas, lead to caking in thelines conducting the gas. Moreover, these harmful components enter theraw meal pre-heating system together with the rotary kiln exhaust gaswhere they precipitate on the raw meal and are reintroduced into therotary kiln with the pre-heated raw meal. A highly accumulating cycle ofharmful substances can thus be formed in the burning process. In orderto avoid this disadvantage, it is conventional to draw off a portion ofthe hot rotary kiln exhaust gases from the burning process via a bypassand to discard it. The significant heat content of the thermally highlyvaluable exhaust gas of the rotary kiln is, however, lost from theburning process by this technique. As a result, the entire burningprocess can become uneconomical. Particularly in very large cementmanufacturing systems, it can no longer be justified in terms of heatefficiency to discard too much rotary kiln exhaust gas containingharmful substances without exploiting its heat content. By so doing, themanufacturing costs of cement, given today's energy costs and those tobe expected in future, become too high.

SUMMARY OF THE INVENTION

In the inventive method only the smallest possible part of fuel isburned in the rotary kiln. The exhaust gas resulting therefrom is assmall as possible and is drawn off by a bypass in an amount ranging from0 through 100% before its use for raw meal pre-heating and/or raw mealcalcination.

In the inventive method, a high degree of calcination of the raw meal iscarried out outside of the rotary kiln in the calcinating stage so thatthe heat energy to be supplied to the rotary kiln is as small aspossible. This smallest possible amount of heat energy to be supplied tothe rotary kiln is produced by burning the smallest possible amount offossil fuel in the rotary kiln. As a result, the smallest possibleamount of combustion gases arise in the rotary kiln. By so doing, therotary kiln can be built relatively short without a negative influenceon the heat transfer between gas and material. Investment costs are thussaved; on the other hand, due to the reduction of the amount of rotarykiln exhaust gas, the concentration of the volatilized harmfulsubstances such as alkali compounds and sulfur in the exhaust gasincreases. For this reason alone, as well as because of the reducedamount of exhaust gas, whose heat content is likewise reduced, thepartial or complete removal and rejection of this rotary kiln exhaustgas via a bypass can be economically justified.

The heat efficiency of the inventive method results from, and theremoval of the rotary kiln exhaust gas from the burning process is aboveall profitable when, a greater percentage of the entire amount of rotarykiln exhaust gas is drawn off via a bypass as this amount of rotary kilnexhaust gas is made smaller. At the same time, the dimensions of therotary kiln can be reduced all the more since less fossil fuel is burnedin the rotary kiln.

To further increase the heat efficiency of the inventive method, andfurther reduce the amount of rotary kiln exhaust gas, at least one partof the fossil fuel burned in the rotary kiln can be replaced by means ofheat generators which cause no exhaust gas in the rotary kiln. By sodoing, the concentration of harmful substances in the rotary kilnexhaust gas increases even further and the heat losses due to therejection of this exhaust gas become even smaller. This method will beparticularly profitable if a significant part of the total amount ofrotary kiln exhaust gas, approximately between 50 through 100%, is drawnoff via a bypass and is removed from the burning process. Since theamount of exhaust gas from the rotary kiln has been even furtherreduced, the dimensions of the rotary kiln can be made even smaller.

The heat generators causing no exhaust gas in the rotary kiln canconsist, for example, of electrical heating elements such as resistanceor induction heaters, transmitters of high-energy rich beams such asbeams of accelerated electrons for irradiation of the material, solarenergy heaters or the like.

An improved burning system for the manufacture of mineral burningproducts, such as cement clinker from raw material, has a raw mealpre-heater, a calcinator which produces a high-degree of calcination ofthe raw meal, a rotary kiln and a clinker cooler. The ratio of thelength of the rotary kiln to its inside diameter is smaller than 14:1and preferably lies in a range of 7:1 through 11:1. A bypass line forthe removal of rotary kiln exhaust gas is connected at the materialintake end of the kiln to the rotary kiln exhaust gas channel leading tothe calcinator or, respectively, raw meal pre-heater. This inventiveburning system, thus, has a bypass line for the removal of rotary kilnexhaust gas. A rotary kiln with a relatively very small specific kilnlength may thus be used. The use of the bypass line is all the moreeconomically justifiable the shorter the rotary kiln is and the lessfossil fuel is burned in the rotary kiln. It is precisely the intensivethermal and chemical preparation of the material for sintering andclinkering in the rotary kiln which allows the ¢working length" of therotary kiln to be significantly shortened, insofar as this length isrequired for the preparation for the sintering and, thus, tosignificantly shorten the over-all rotary kiln. Said preparation isattainable with the assistance of a high-degree of precalcination.

Thus, surprisingly, it is possible to reduce the specific kiln length byapproximately one-half with respect to the value standard up to now in arotary kiln which is designed for modern dry processes for burningintensely pre-calcinated material into products of the burning processsuch as cement clinker. Expressed in numbers, the ratio of kiln lengthto the inside kiln diameter in such a rotary kiln is reduced from theprevious approximately 16:1 to 8:1. A few advantages of the inventiveburning system with short rotary kiln with respect to a system withtraditional rotary kiln are the following:

Less kiln raw material, less kiln brick lining subject to wear, lesssurface radiating heat, therefore lower heat losses, lower weight,therefore also the need of lower driving power, and heat expansion is nolonger such a problem. With less heat expansion, more favorableconditions exist for kiln seatings and seals.

In all, lower investment and operating costs of the burning systemthereby ensue along with the advantage that, as a result of keeping theamount of exhaust gas as small as possible, a partial or completeremoval of this exhaust gas from the burning process can now beeconomically justified.

Alternately, it has been found advantageous to incorporate non-fossilfuel heat generators in the calcining unit. These additional heatgenerators can supply about 20% of the total heat requirement of thecalcining unit. The remaining 80% can be supplied by the fossil fuelburning elements in the calcining unit.

The use of non-fossil fuel heating elements in the calcining unitcontributes to a saving of valuable fossil fuel as well as aminimization of the overall dimensions of the rotary kiln. Infraredradiators or electric heating units can be used in the calciningsection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a burning system for cement clinker manufacture witha short rotary kiln and bypass for rotary kiln exhaust gas;

FIG. 2 illustrates an embodiment of the burning system that is differentfrom that of FIG. 1;

FIG. 3 illustrates the short rotary kiln of the burning system of FIG. 1in a longitudinal section and in enlarged representation;

FIG. 4 illustrates a short rotary kiln designed differently from that ofFIG. 3; and

FIG. 5 is a schematic diagram of a burning or sintering installationincorporating non-fossil fuel heat generators in the calcining unit aswell as in the rotary kiln.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Not by way of limitation, but by way of disclosing the best mode ofpracticing our invention and by way of enabling one of ordinary skill inthe art to practice my invention, there are disclosed in FIGS. 1-5alternate embodiments of our invention.

The burning system of FIG. 1 exhibits a rotary kiln 10 to which a rawmeal pre-heater 11 and a calcinator 12 are pre-connected and to which aclinker cooler 13 is post-connected. Cement raw meal 14 flows from thetop toward the bottom through the pre-heater 11 and calcinator 12 in acombined counterflow/direct flow to the hot exhaust gases leaving therotary kiln 10 and/or to the hot exhaust air of the cooler 13. Theseexhaust gases are drawn off by means of the induced-draught blower 15.The raw meal pre-heater 11 consists of cyclone heat exchangers 16, 17,18. To induce the intense or high-degree of calcination of the cementraw meal before the meal enters into the rotary kiln 10, fuel B₁ issupplied between the cyclone heat exchangers 18 and 19 and fuel B₂ issupplied between the heat exchangers 19 and 20 as seen in the directionof flow of the cement raw meal. In the first burning stage or fuelburning point B₁, approximately 30% of the calcining work is carriedout, approximately 70% in the second burning stage B₂. Preferably, thetwo burning stages utilize various types of inferior fuels. In the lowerburning stage, fuel B₂ of every type is burned in a gas atmosphere thatis formed from the cooler exhaust air supplied via the tertiary air line21 and, under certain conditions, of rotary kiln exhaust gas. The fuelB₂ and gas from line 21, upon intimate mixing with the raw mealpre-heated in the raw meal pre-heater burns in such a manner that theheat or combustion is directly communicated to the raw meal and isemployed to produce the desired intense or high-degree of calcining. Abypass line 22 for the removal of rotary kiln exhaust gas containingharmful substances is arranged in the rotary kiln exhaust gas channel tothe calcinator 12 or, respectively, raw meal preheater 11. The highlycalcined cement raw meal enters into the rotary kiln 10 through line 23.

As shown in FIG. 3, the rotary kiln 10 has a ratio of the length L tothe inside diameter D of approximately 8:1 and therefore has anunusually small specific kiln length.

A burning location or point 24a in which fossil fuel B₃ is burned islocated at the discharge end of the rotary kiln 10 and is supplied via aprimary air line 24. An additional supply of fuel B₄ particularly inlumpy form can be provided at a fuel burning point near the materialintake end of the rotary kiln 10 via line 25. The solid fuel ispreferably composed of unground coal pieces which quasi-swim on the rawmeal and burn in the raw meal almost without flame, whereby theefficiency of the heat transfer is very high. The finished, burnedcement clinker is cooled in the clinker cooler 13. The cooled materialleaves cooler 13 via line 26. Fresh air streams into the cooler throughline 27; a part of the cooler exhaust air is drawn off via line 28.

Since the inventive short rotary kiln 10 has a comparatively smallspecific kiln length, it can be seated on only two seating locations 29and 30. The drive of the rotary kiln is indicated with 31.

The rotary kiln 10a of FIG. 4 has a greater interior diameter in thearea of its material introduction or, respectively, in its sinteringpreparation zone than in the remaining rotary kiln longitudinal area. Inthe area with expanded diameter, the interior walls of the rotary kilnexhibit ceramic lifting installations 31a for lifting and scattering thematerial to be sintered, whereby the heat transfer between rotary kilnexhaust gas and material is intensified. Moreover, the dwell time of thematerial in the rotary kiln is increased with a simultaneous reductionof the gas velocity due to the fact that the sintering preparation zoneis expanded in cross section. As a result, the heat transfer between gasand material is likewise improved.

In the burning system of FIG. 2, parts coinciding with FIG. 1 areprovided with the same reference numerals. A mixing chamber 33 in whichhot rotary kiln exhaust gas of approximately 1300° C. is very quicklycooled to approximately 400° through 600° C. by admixture of cold airsupplied via blower 34 and/or addition of water and/or raw meal isarranged above the kiln intake head 32 of the rotary kiln 10.

The cooled rotary kiln exhaust gas is drawn off via the bypass line 22.Preferably, approximately 20 through 100% of the total amount of rotarykiln exhaust gas is drawn off via the bypass. This percentage of theamount of exhaust gas is all the greater the smaller the total amount ofrotary kiln exhaust gas is. The 20% through 100% of the exhaust gasesdrawn off via the bypass line 22 is cooled from about 1300° C. to 400°C. through 600° C. by the addition of cold air.

The remaining amount of rotary kiln exhaust gas is conducted into thelowest cyclone 20 of the cyclone heat exchanger system via the ascendingline 35 of the calcinator 12 and/or via a line 36. The stream ofmaterial 37 leaving the cyclone 19 is divided into two streams by adistribution element which is not illustrated. One partial stream of thematerial is conducted into the ascending line 35 of the calcinator 12designed as a burning segment. The other partial stream of the materialis introduced into the exhaust gas line 36. The rotary kiln exhaust gasin line 36 is cooled by the partial stream of the material to such adegree that this rotary kiln exhaust gas as well as the exhaust gas inthe calcining device 12 exhibit an approximately identical temperatureof approximately 800° through 900° C. upon their mixing in cyclone 20.In this manner, no kiln exhaust gas containing harmful matter arrives inthe descending line 35 of the calcinator 12 designed as a burningsegment.

The distribution of the amount of rotary kiln exhaust gas not drawn offby the bypass line 22 to the exhaust gas line 36 and/or to the ascendingline 35 of the calcinator 12 can be accomplished by means of regulatingelements 38, 39 or, respectively, 41 in lines 36, 21 or, respectively,22.

The burning location 24a or, respectively, the burning locations 24a and25a of the rotary kiln 10 which produce exhaust gases in the rotary kiln10 are at least partially replaced in the sample embodiment of FIG. 2 bymeans of heat generators arranged on or, respectively, in the rotarykiln. These heat generators cause no exhaust gases in the rotary kilnand are symbolically illustrated by means of many small arrows with thedesignation B₅. A numerical example follows.

Of the heat energy to be supplied to the total burning process, 65% mustbe supplied to the pre-heater 11 and the calcinator 12 and the remaining35% must be supplied to the rotary kiln 10. Up to now, this 35% heatrequirement in the rotary kiln 10 was provided by means of the singleburning location 24a. In order to reduce the amount of rotary kilnexhaust gas and, thus, also the heat loss that arises due to the removalof rotary kiln exhaust gases containing harmful substances via thebypass 22, the amount of the fossil fuel B₃ can be inventively reducedto such a degree that it provides only 15% of the heat requirement inthe rotary kiln instead of 35%; the heat generators B₄ and B₅ produce noexhaust gases in the rotary kiln 10, and each provides the remainingheat requirement of, for example, respectively 10%, so that the total35% heat energy is then generated in the rotary kiln.

The heat generators B₅ are, for example, electric heating elements suchas resistance or induction heaters which are built into the fireprooflining of the rotary kiln and keep the interior wall temperature at, forexample, 1200° C. The rotary kiln 10 is advantageously equipped with aheat insulation 40. Upon the reduction of the fuel B₃ for the burninglocation 24a, a transmitter B₆ of high-energy rich beams can be arrangedin its area, which beams then replace the missing heat requirement ofthe rotary kiln by means of radiation chemical treatment of thematerial. Such a transmitter 25a can also be arranged at the materialintake side of the rotary kiln.

The amount of fossil fuel B₃ supplied to the rotary kiln 10 should notbe completely reduced to zero. A certain amount of exhaust gas in thekiln 10 is necessary. The volatilizing harmful components such as alkalicompounds and sulfur condense on the dust particles suspended in theexhaust gas in the rotary kiln 10.

It will be understood that the invention is also employable in burningsystems in which the gas and material are conducted in the pre-heaterand calcinator in two or more series parallel to one another.

Additionally, it has been found that the heat energy supplied by theburners or burning locations B₁, B₂ in the calciner 12 of FIG. 1 or theburning locations or burners B₂ in the calciner 12 of FIG. 2 can bepartly replaced by heating elements that do not produce exhaust gases.FIG. 5 which is an installation corresponding to FIG. 2 shows anadditional set of heat generators B₇ arranged on or in calciner 12. Theheat generators B₇ produce no exhaust gases in calciner 12. Heatgenerators B₇ can be used in combination with heat generators B₅ or B₆.Alternatively, heat generator B₇ can be used without heat generators B₅,B₆.

Of the total heat energy required by the calciner 12 approximately 20%can be supplied by heat generators B₇. The remaining 80% of thenecessary heat energy to be supplied to the calciner 12 can be producedby the combustion of fossil fuels in burners or burning elements B₁, B₂,or B₂ alone. The exhaust gases from the fossil fuels contribute tokeeping the raw meal passing through the calciner 12 in suspension.

The additional heat generators B₇ can be used with kilns shown in FIGS.3, 4, as well as in the installation of FIG. 1. Infrared radiators canbe used as the heat generators B₄ -B₇. Available infrared heaters canheat the irridiated material to approximately 1000° C. and higher.Electric heating units such as resistance or induction heaters can alsobe used. High energy particle beams, produced by an appropriate source,such as accelerated electrons, can be used as heat generators B₄ -B₇, toirridiate and heat the raw meal being calcined. Solar heaters can alsobe used for heating elements B₄ -B₇.

Although various modifications might be suggested by those skilled inthe art, it should be understood that we wish to embody within the scopeof the patent warranted hereon all such modifications as reasonably andproperly come within the scope of our contribution to the art.

We claim as our invention:
 1. In a combustion installation for theproduction of mineral combustion products such as cement clinkers out ofraw meal having a raw meal preheater, a rotary kiln and a clinkercooler, an improvement comprising:a calciner having an input connectedto the output of said preheater and an output connected to the input ofsaid rotary kiln; fossil fuel burning means operatively connected tosaid calciner to partly heat and partly calcine the raw meal passingtherethrough; and non-combustion heating means operatively connected tosaid calciner to provide additional heat to the raw meal passingtherethrough without introducing any additional gases into saidcalciner.
 2. The installation of claim 1, including further:secondfossil fuel burning means operatively connected to said kiln to partlyheat and sinter the material passing therethrough; and second heatingmeans operatively connected to said kiln to provide additional heat tothe material passing therethrough without introducing any additionalgases into said kiln.
 3. The installation of claim 1, wherein saidheating means comprises:electric heating elements consisting ofresistance heating units.
 4. The installation of claim 1, wherein saidheating means comprises:a source of high energy particles adapted toirradiate the raw meal passing through said calciner.
 5. Theinstallation of claim 1, wherein said heating means comprises:a sourceof infrared radiation adapted to irradiate the raw meal passing throughsaid calciner.
 6. The installation of claim 1, wherein said heatingmeans comprises:a source of heat generated by solar energy adapted toheat the raw meal passing through said calciner.
 7. The installation ofclaim 1, wherein said calciner has two stages with said fossil fuelburning means comprising first and second heating locations with onelocation in each of said two stages.
 8. The installation according toclaim 1, wherein:said fossil fuel burning means is adapted to generateon the order of 80% of the heat needed for said calciner; and saidheating means is adapted to generate the remaining heat needed for saidcalciner.
 9. The installation according to claim 2, wherein:said secondheating means consisting of resistance heaters.
 10. The installationaccording to claim 2, including a third heating means affixed to aninput end of said kiln;said third heating means consisting of resistanceheaters.
 11. The installation according to claim 2, including a fourthheating means affixed to an output end of said kiln;said fourth heatingmeans consisting of resistance heaters.
 12. The installation of claim 2wherein said second heating means consists of induction heaters.
 13. Theinstallation of according to claim 2 wherein said second heating meansconsists of infrared heaters.
 14. The installation according to claim 2wherein said second heating means consists of solar heaters.
 15. Theinstallation according to claim 2 wherein said second heating meansconsists of sources of high energy particle beams.
 16. The installationof claim 2 including a third heating means affixed to an input end ofsaid kiln and consisting of induction heaters.
 17. The installation ofclaim 2 including a third heating means affixed to an input end of saidkiln and consisting of infrared heaters.
 18. The installation of claim 2including a third heating means affixed to an input end of said kiln andconsisting of solar heaters.
 19. The installation of claim 2 including athird heating means affixed to an input end of said kiln and consistingof sources of high energy particle beams.
 20. The installation of claim2 including a fourth heating means affixed to an output end of saidkiln, said fourth heating means consisting of induction heaters.
 21. Theinstallation of claim 2 including a fourth heating means affixed to anoutput end of said kiln, said fourth heating means consisting ofinfrared heaters.
 22. The installation of claim 2 including a fourthheating means affixed to an output end of said kiln, said fourth heatingmeans consisting of solar heaters.
 23. The installation of claim 2including a fourth heating means affixed to an output end of said kiln,said fourth heating means consisting of sources of high energy particlebeams.
 24. The installation of claim 1 wherein said heating meanscomprises inductive heating means.
 25. In a combustion installation forthe production of mineral combustion products such as cement clinkersout of raw meal comprising:a raw meal preheater; a calciner receivingpreheated meal from said preheater; a rotary kiln having an input endreceiving calcined meal from said calciner, and an output end; a clinkercooler receiving clinker from the output end of said rotary kiln; fossilfuel burning means operatively connected to said kiln to partly heat andsinter the material passing therethrough; first heating means affixedalong the length of said kiln between said input end and said outputend; second heating means affixed at said input end of said kiln; thirdheating means affixed at said output end of said kiln; said heatingmeans being adapted to provide additional non-combustion type heat tothe material passing through said kiln without introducing additionalgases therein.